Plasma devices are used to form ions, electrons, and dissociated neutral atoms from a gas. The dissociated ions and neutral atoms are then used to remove material from a work sample, grow material on a work sample or implant ions in a work sample. Two primary methods are used for producing the plasma. The first method uses electrodes which are arranged in the plasma chamber walls. These electrodes are electrically charged to produce ions and electrons from the gas within the plasma chamber. A device of this type is disclosed in EP 0284867A2.
The second primary method for producing plasma uses a magnetic field in conjunction with microwave energy. This method produces plasma from the working gas by the interaction of a magnetic field and an electric field. The electric field is associated with microwaves which enter the device chamber from a microwave waveguide. The electric field associated with the microwaves is perpendicular to the propagation path of the microwaves. If a magnetic field is provided at a right angle to the electric field, the electric field and the rotation of plasma electrons around the magnetic field will be in phase at some point. This point is referred to as the plane of resonance. At the plane of resonance the microwave electric field is constantly accelerating the electrons rotating about a magnetic field. This acceleration imparts sufficient energy to the electrons to dissociate molecules of the working gas into atoms and to remove electrons from the atoms to create ions and additional electrons. These ions and neutral atoms then travel in the direction of the magnetic field and impinge upon the work surface.
In this method of plasma production, magnetic fields are produced by one of two methods. The first method requires the use of electromagnets. These magnets are usually in the shape of a solenoid which encloses most or part of a plasma excitation chamber. The circulation of current through the magnets can produce excessive heat. The circuitry to control the strength of the magnetic fields generated by the energized coils and the considerable size and weight of such electromagnets increase the costs of these systems.
To eliminate the disadvantages of electromagnets, permanent magnets have been used. U.S. Pat. No. 4,433,228, for example, discloses a microwave plasma source which places a permanent magnet within a microwave waveguide. While electrical power consumption is reduced by replacing the electromagnet with a permanent magnet, this placement of the magnet results in the heating of the magnetic material by the microwaves. This heating reduces the effective life of such magnetic material. Additionally, the electric field associated with the microwaves is rotated once it passes through the magnetic material and the electric field strength is attenuated by the magnetic material. Adjusting the resonance plane for the production of the plasma is also difficult since the magnetic material is within the plasma chamber or the microwave waveguide.
Another plasma producing device which uses permanent magnets is disclosed in European Patent Application 0286132A2. This plasma producing device requires multiple magnets. These multiple magnets are necessary to produce a magnetic field which projects downwardly into the container. These multiple magnets are arranged in two layers of opposing polarity. The opposing polarities within proximity of one another produce magnetic flux lines within the container. These flux lines produce a downward component of a magnetic field within the container. This downward component then interacts with the electric field of the microwave energy to produce the plasma from the working gas.
The requirement of multiple permanent magnets increases the cost of these plasma producing devices. The use of multiple magnets also make adjustments in the placement of the plane of resonance more difficult since more than one magnet must be moved. Finally, the multiple magnet structure requires more magnetic material to produce a magnetic field of the same strength in the plane of resonance as would a more centrally located single magnet.